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(American Journal of Botany. 1999;86:1154-1159.)
© 1999 Botanical Society of America, Inc.

Genotypic variation for condensed tannin production in trembling aspen (POPULUS TREMULOIDES, salicaceae) under elevated CO2 and in high- and low-fertility soil1

Jennifer L Mansfield 2 , Peter S Curtis 2,3, 6, Donald R Zak 3,4 and Kurt S Pregitzer 3,5

2Department of Plant Biology, The Ohio State University, Columbus, Ohio 43210; 3University of Michigan Biological Station, Pellston, Michigan 49769; 4School of Natural Resources and Environment, University of Michigan, Ann Arbor, Michigan 48109; and 5School of Forestry and Lake Superior Ecosystems Research Center, Michigan Technological University,Houghton, Michigan 49931

Received for publication April 30, 1998. Accepted for publication December 18, 1998.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
The carbon/nutrient balance hypothesis suggests that leaf carbon to nitrogen ratios influence the synthesis of secondary compounds such as condensed tannins. We studied the effects of rising atmospheric carbon dioxide on carbon to nitrogen ratios and tannin production. Six genotypes of Populus tremuloides were grown under elevated and ambient CO2 partial pressure and high- and low-fertility soil in field open-top chambers in northern lower Michigan, USA. During the second year of exposure, leaves were harvested three times (June, August, and September) and analyzed for condensed tannin concentration. The carbon/nutrient balance hypothesis was supported overall, with significantly greater leaf tannin concentration at high CO2 and low soil fertility compared to ambient CO2 and high soil fertility. However, some genotypes increased tannin concentration at elevated compared to ambient CO2, while others showed no CO2 response. Performance of lepidopteran leaf miner (Phyllonorycter tremuloidiella) larvae feeding on these plants varied across genotypes, CO2, and fertility treatments. These results suggest that with rising atmospheric CO2, plant secondary compound production may vary within species. This could have consequences for plant–herbivore and plant–microbe interactions and for the evolutionary response of this species to global climate change.

Key Words: carbon dioxide • condensed tannins • global change • plant–herbivore interaction • Populus tremuloides • Salicaceae


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
Atmospheric CO2 is predicted to double within 100 yr (Houghton, Jenkins, and Ephraums, 1995 ), yet the consequences of this perturbation for plant secondary compound synthesis and plant-herbivore interactions are not fully understood (Ayres, 1993 ; Lindroth, 1996 ). The carbon/nutrient balance hypothesis (CNBH; Bryant, Chapin, and Klein, 1983 ; Waterman, Ross, and McKey, 1984 ) predicts that as green leaf carbon to nitrogen ratio (C/N) increases, as is often observed at high CO2 (Ceulemans and Mousseau, 1994 ), production of carbon-rich secondary compounds such as tannins will also increase. While studies involving leaf C/N manipulation through alteration of light and/or nutrient availability have generally supported the CNBH (e.g., Larsson et al., 1986 ; Bryant et al., 1987 ; Nichols-Orians, 1991 ; Dudt and Shure, 1994 ,) results using elevated CO2 have been more variable. For example, Lincoln and Couvet (1989) found that monoterpenes did not increase in Mentha piperita L. cv. Mitcham grown under elevated CO2. In contrast, Fajer, Bowers, and Bazzaz (1992) found that Plantago lanceolata L. grown at 70 Pa CO2 did not have increased carbon-based secondary compounds compared to ambient-CO2-grown plants, and levels actually declined in some cases.

Fewer studies have examined the phytochemical response of woody plants to elevated CO2, although results more consistently support the CNBH. In Populus tremuloides Michx., Acer saccharum Marsh. (Lindroth, Kinney, and Platz, 1993 ), Betula papyrifera Marsh. (Lindroth, Arteel, and Kinney, 1995 ), and Betula pendula Roth. (Lavola and Julkunen-Tiitto, 1994 ) condensed tannin concentration increased in elevated- compared to ambient-CO2-grown plants. For example, Lindroth, Arteel, and Kinney (1995) found that tannin concentration in B. papyrifera doubled under twice ambient CO2 conditions, and elevated CO2 treatment in low-fertility soil resulted in a 60% increase in tannin concentration of B. pendula leaves (Lavola and Julkunen-Tiitto, 1994 ). Other secondary compounds showed more variable responses as the phenolic glycoside salicortin increased 55% in P. tremuloides, whereas ellagitannin decreased in Quercus rubra L. and increased in A. saccharum grown under elevated CO2 (Lindroth, Kinney, and Platz, 1993 ). In B. pendula, phenolic glycoside production was variable across CO2 treatments (Lavola and Julkunen-Tiitto, 1994 ).

Many plant species exhibit significant intraspecific variation in herbivore resistance, some of which is related to differences in production of defensive compounds (Lege, Smith, and Cothren, 1992 ; Hemming and Lindroth, 1995 ; Hwang and Lindroth, 1997 ). However, little is known about the magnitude of genetic variation within plant populations for secondary compound production under elevated CO2. Differential genotypic responses in the synthesis of defensive compounds could have evolutionary consequences; for instance, genotypes that allocate more carbon to defense compounds may experience a selective advantage in a CO2-enriched atmosphere (Geber and Dawson, 1993 ). In the only study to date examining this issue, Fajer, Bowers, and Bazzaz, (1992) found no significant differential responses of secondary compound production under elevated CO2 among six clones of Plantago lanceolata. In light of documented genetic variation in other responses to elevated CO2 (e.g., reproduction and stomatal index in Raphanus raphanistrum L.; Curtis, Snow, and Miller, 1994 ; Case, Curtis, and Snow, 1998 ) further study of this question is clearly warranted.

Here, we present data on genotypic variation for condensed tannin production in trembling aspen (Populus tremuloides) grown under elevated and ambient CO2 and two soil nutrient levels throughout one growing season. We hypothesized that, in accordance with the CNBH, plants grown under elevated CO2 and low nutrients would have the highest levels of condensed tannin, while plants grown under ambient CO2 and high nutrients would have the lowest levels. In addition, given high levels of clonal variation in morphology (Barnes, 1959 ), vegetative growth (Sakai and Burris, 1985 ) and secondary chemistry (Lindroth and Hwang, 1996a ) within the source population and the sensitivity of tannin production by P. tremuloides to environmental conditions (Lindroth and Hwang, 1996b ), we expected that genotypes would respond differently to elevated CO2, with some genotypes increasing tannin production and others decreasing production or showing no change. Finally, we predicted that naturally occurring levels of lepidopteran herbivore damage would reflect treatment and genotype effects on tannin production.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 LITERATURE CITED
 
This experiment was part of a larger study of Populus tremuloides (Salicaceae) responses to elevated CO2 conducted at the University of Michigan Biological Station (UMBS) in northern lower Michigan (Curtis et al., 1996 ). During the fall of 1993, roots from six different male genotypes (clones) were collected from Pellston Plain, Michigan, within 5 km of the elevated CO2 study site. The rooted cuttings propagated from these samples were planted in 3 m diameter x 2.4 m tall open-top CO2 chambers (Heagle et al., 1989 ) in the spring of 1994. Each chamber contained two individuals of each aspen genotype.

The chambers were positioned over open-bottom root boxes that had been filled with either low- or high-fertility soil. High-fertility soil was 100% locally derived Kalkaska series topsoil, while low-fertility soil was a mixture of 20% Kalkaska topsoil and 80% of the C horizon of a Rubicon sand (the dominant material underlying the study site). In a previous experiment using the same soil mixtures, nitrogen mineralization was significantly higher in the high-fertility treatment (348 µg N·g-1·d-1) than in the low-fertility treatment (45 µg N·g-1·d-1) (Randlett et al., 1996 ). Half of the chambers were maintained at ambient CO2 (seasonal 24-h average = 35.6 Pa), and half were maintained at elevated CO2 (seasonal 24-h average = 70.7 Pa). The experiment was arranged as a two-way randomized complete-block split-plot design with CO2 and soil fertility crossed within each of five replicate blocks (20 chambers total). Genotype was considered a subplot within each treatment combination (the main plot).

Leaf samples were collected on three dates in 1995: 22 June, 6 August, and 13 September. In each chamber, the youngest fully expanded leaf from one individual of each genotype was clipped at the petiole, flash frozen in liquid nitrogen and stored on dry ice until it was placed in a -82°C freezer. On the last sampling date, leaves were not flash frozen, but rather were placed immediately on dry ice. After freezing, entire leaf samples minus the petioles were lyophilized and pulverized to a fine powder. Leaves also were sampled on 8 August from mature individuals of the same genotypes growing in Pellston Plain. On 29 June, leaves were sampled as above and C/N determined by CHN Analysis (Perkin-Elmer Model 2400, Perkin-Elmer Corp., Norwalk, Connecticut).

Condensed tannins were assayed by radial diffusion (Hagerman, 1987 ) calibrated against purified P. tremuloides tannin (Hagerman and Butler, 1980 ; R. Lindroth, personal communication, University of Wisconsin). Approximately 100 mg of leaf tissue were extracted with 70% acetone and the supernatant applied to wells within agar plates containing bovine serum albumin. The binding of protein with tannin resulted in precipitation rings whose diameter was proportional to tannin levels in the sample. In our assays, ring diameter ranged from 0.57 to 1.29 cm. Reference standards were established for each batch of agar prepared, and r2 values for the calibration curves were always >0.98.

In 1995, P. tremuloides throughout the UMBS area, including chamber-grown plants, were attacked by the lepidopteran leaf miner Phyllonorycter tremuloidiella Braun (aspen blotch miner). This species excavates a circular section of leaf mesophyll, allowing us to measure CO2 and soil N effects on herbivore leaf consumption and larval performance. For three of the aspen genotypes, single leaves on which the miner had reached pupal stage were collected at random from separate chambers and the leaf dry mass consumed (DWc) was calculated by

where Am was the total area of mesophyll consumed, LMAm was the leaf mass per area of the mined tissue, and LMAu was the leaf mass per area of an adjacent unmined section of the same leaf. Pupae were dissected from the leaf, dried, and weighed.

Overall treatment effects were analyzed using repeated-measures analysis of variance for split-plot design (Gumpertz and Browne, 1993 ). Within a sampling date, elevated vs. ambient response means within a fertility level were compared by least significant difference (a priori comparisons), while comparisons across fertility levels, among clones, and to mature plants were by the minimum significant range (a posteriori comparisons; Sokal and Rohlf, 1981 ).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 LITERATURE CITED
 
Leaf tannin concentration was affected by both CO2 and fertility (the main plot effects) and by genotype (the subplot effect; Table 1). At each sampling date, tannin concentration was highest in the elevated-CO2, low-fertility treatment and lowest in the ambient-CO2, high-fertility treatment (Table 2). This pattern of response paralleled leaf C/N (Table 3) and was consistent with predictions of the CNBH. Within a soil fertility treatment, tannin concentration was significantly higher in elevated compared to ambient-CO2-grown plants, increasing an average of 76% across dates at high soil fertility and 22% at low soil fertility (Table 2). Within a CO2 treatment, tannin concentration was also significantly higher in low- compared to high-fertility soil. Leaf C/N showed similar variation, increasing 35% due to CO2 (across fertility treatments) and 29% due to soil fertility (across CO2 treatments) (Table 3).


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  		Table 1. Results of repeated-measures ANOVA for tannin production in Populus tremuloides. Significant CO2, fertility, and genotype effects are indicated on the between-subjects portion of the table, while significant effects through time are shown on the within-subjects portion of the table

 

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  		Table 2. Effects of CO2 and soil fertility on Populus tremuloides leaf tannin concentration in June, August, and September. Plants were grown for 2 yr at ambient or elevated atmospheric CO2 and in soils of either high- or low-N availability. Mean (± 1 SE), N = 5

 

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  		Table 3. Effects of CO2 and soil fertility on Populus tremuloides leaf C and N content. Plants were grown for 2 yr at either ambient or elevated CO2 and in soils of high- or low-N availability. Mean (± 1 SE), N = 5 for each treatment

 
There was significant variation in leaf tannin concentration within and among aspen genotypes as a function of time, CO2, and fertility (Table 1, Fig. 1). For clarity, only June and August data are shown in Fig. 1. September data parallel those of August. Overall, genotype 61 produced the least tannin (54.3 mg/g averaged across treatments and sampling dates), whereas genotype 8 produced the most (138.8 mg/g averaged across treatments and sampling dates). Genotypes generally responded to treatments in a manner consistent with the CNBH, with tannin concentration increasing under elevated CO2 and low soil fertility compared to ambient CO2 and high soil fertility, but there were notable exceptions. For example, genotypes 42, 51, and 61 showed strong responses to CO2 under both low and high soil fertility, whereas CO2 responses were restricted to high soil fertility in genotypes 2 and 8. This variation led to a significant CO2 x genotype interaction for tannin concentration (Table 1). In contrast, there was no fertility x genotype interaction, indicating a consistent increase in tannins at low soil fertility across the six genotypes. There were also no significant changes in the rank order of genotypes with respect to leaf tannin concentration.



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  		Fig. 1. Leaf tannin concentration in six genotypes of Populus tremuloides sampled from 2-yr-old saplings growing in field open-top chambers (bars) or from mature clones growing in Pellston Plain (open circles). Chamber-grown individuals were growing in soils of low (L) or high (H) N fertility and under ambient (open bars) or elevated (shaded bars) CO2 levels. For clarity only June and August data are shown. Vertical bars indicate 1 SE. Significance tests were within a fertility level, where + P < 0.08, * P < 0.05, ** P < 0.01

 
Leaf tannin concentration also varied seasonally; August levels were consistently lower than those measured in June or September (Table 2). This variation was genotype specific, however, with some genotypes showing little decline in August (e.g., genotype 2) and others showing as much as a 43% decline in this period across all treatments (e.g., genotype 42) (Fig. 1). The temporal change also was strongest at low soil fertility and was reflected in significant time x genotype and time x genotype x fertility interactions (Table 1).

There was no significant difference in leaf tannin concentration between native plants growing on the Pellston Plain (66.1 ± 14.8 mg/g) and those in our experiment grown under ambient CO2 and low soil fertility (77.4 ± 13.2 mg/g, t test). Only genotype 8 differed significantly in leaf tannin between locations (Fig. 1).

Pupal dry mass of P. tremuloidiella that fed on chamber-grown plants was marginally greater in elevated compared to ambient CO2 treatments (+8%, P < 0.09, Table 4), and both the amount of tissue consumed (P < 0.01) and pupal dry mass (P < 0.07) varied among the three genotypes examined. There were no effects of soil fertility on pupal mass, although larvae consumed 69% more tissue from low-fertility compared to high-fertility treatments (P < 0.05). Carbon dioxide treatment had no effect on the amount of tissue consumed.


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  		Table 4. Leaf dry mass consumption (DWc) and pupal dry mass (DWp) of Phyllonorycter tremuloidiella feeding on Populus tremuloides genotypes grown at ambient or elevated CO2 and in soils of high or low fertility. Mean (±1 SE)

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
LITERATURE CITED
 
Our results suggest that rising atmospheric CO2 will alter patterns of carbon allocation to leaf-condensed tannins in P. tremuloides in a manner consistent with the CNBH. These results are in agreement with recent work reporting increased carbon-based secondary compounds in woody species (Lindroth, Kinney, and Platz, 1993 ; Lavola and Julkunen-Tiitto, 1994 ; Lindroth, 1996 ), but contradict some earlier studies that showed no change in secondary compound production under elevated CO2 (e.g., Lincoln and Couvet, 1989 ; Fajer, Bowers, and Bazzaz, 1992 ; Lincoln, Fajer, and Johnson, 1993 ). For example, Lincoln and Couvet (1989) found no increase in volatile monoterpenes in Mentha piperita cv. Mitcham grown under elevated CO2. Likewise, the perennial herb Plantago lanceolata showed no significant increase in iridoid glycosides or phenylpropanoid glycosides (Fajer, Bowers, and Bazzaz, 1992 ). However, because tannins are immobile compounds that do not have a high turnover rate, their response to CO2 may differ from mobile compounds such as monoterpenes. We also found that the effect of CO2 on leaf tannins was independent of soil N fertility levels. Under conditions of low soil N mineralization, already high leaf C/N was further increased under elevated CO2, leading to a concomitant increase in leaf tannin concentration. However, the relative response to CO2 was greater under high soil fertility conditions, where tannin concentration almost doubled between ambient and elevated CO2.

Consistent with the results of Hwang and Lindroth (1997) , we found that leaf tannin concentration varied among aspen genotypes. However, it is particularly interesting that genotypes responded differently to increased CO2. Some genotypes showed significant increases in tannin production, whereas others did not. Changes in tannin production may impact herbivore success, particularly when coupled with changes in plant tissue quality. Traw, Lindroth, and Bazzaz (1996) found that leaves with less foliar nitrogen and more condensed tannins had poorer gypsy moth (Lymantria dispar L.) larval performances. Similarly, Hwang and Lindroth (1997) reported that clonal variation in tannin production contributed to differential performance of insects feeding on those clones. Genotypes that show little or no increase in tannin production at high CO2 may therefore be more susceptible to herbivore attack or microbial invasion, relative to genotypes showing a more marked increase in tannins. Thus, genotypes with increased tannins may have an advantage in future climates.

We also noted an overall decrease in tannin production in August compared to June and September. Several factors likely contributed to this temporal pattern. For example, the physiological age of the leaves sampled varied among dates. In September, new leaves had stopped forming and the youngest fully expanded leaves likely were more mature than corresponding leaves sampled in August. Condensed tannin concentration in Quercus agrifolia Nee., Q. ilex L., Q. semecarpifolia Sm., Q. serrata Thunb., and Q. glauca Thunb. increased gradually throughout the season as leaves matured (Kleiner, Montgomery, and Schultz, 1989 ; Mauffette and Oechel, 1989 ; Harinder, Dawra, and Singh, 1991 ), indicating that older leaves have more time to accumulate tannins and may thereby become better defended. An increase in self-shading with canopy development could have contributed to lower tannin concentration in newly expanded leaves in August compared to June, a temporal trend also observed by Auerbach and Alberts (1992) . Light levels are positively correlated with foliar secondary compound conentrations in aspen (Lindroth and Hwang, 1996 b).

Under elevated CO2, leaf nitrogen content typically decreases and herbivores often respond with a compensatory increase in consumption (Lincoln and Couvet, 1989 ; Lindroth, Kinney, and Platz, 1993 ). Lincoln, Couvet, and Sionit (1986) found that larvae of the soybean looper (Pseudoplusia includens Walker) increased consumption of elevated-CO2-grown Glycine max L., a response that was negatively correlated with leaf nitrogen concentration. Most work on the responses of herbivores to high-CO2-grown plant tissue has been conducted with controlled feeding studies under artificial growth conditions. We collected herbivory data from a naturally occurring population of P. tremuloidiella feeding on plants in the open-top chambers. Soil fertility clearly influenced consumption by this herbivore; 43% more dry mass was removed in low-fertility- compared to high-fertility-grown plants. However, soil fertility did not affect pupal dry mass, indicating that P. tremuloidiella successfully compensated for low nutritional quality through increased consumption. Elevated CO2 effects on P. tremuloidiella preference and performance was less clear. There was a trend toward greater pupal dry mass following consumption of elevated-CO2-grown tissue but no effect on the amount of tissue consumed. It is important to note that small sample sizes, particularly at high CO2 and low soil fertility, limit our ability to draw firm conclusions about the magnitude of the CO2 effect. Traw, Lindroth, and Bazzaz (1996) found gypsy moth pupal dry masses decreased when reared on elevated-CO2-grown Betula alleghaniensis Britton leaves but did not change when reared on high-CO2-grown Betula populifolia Marsh.

We found that plant genotype significantly affected both leaf dry mass consumed and leaf tissue chemistry. However, consistent with other studies, we did not find a significant correlation between leaf tannin levels and herbivore performance. Lindroth, Kinney, and Platz (1993) and Hemming and Lindroth (1995) found no relationship between condensed tannin levels and gypsy moth performance. These results suggest that variation in other defensive compounds such as phenolic glycosides or in morphological traits such as leaf toughness could have contributed to variation among clones in leaf miner preference. Auerbach and Alberts (1992) examined factors contributing to host perference in P. tremuloideilla and concluded that differences in phenology among P. tremuloides, P. grandidentata, and P. balsamifera could best account for observed differences in feeding on these three species. There was, however, little variation in tannin content among these potential hosts.

Open-top chamber-grown plants often differ in numerous respects from field-grown individuals and we therefore were interested in the extent to which leaf tannin levels in our experiment plants resembled those of their progenitor clones growing in a natural forest ecosystem. We found no significant difference in mean leaf tannin concentration in plants from low-fertility, ambient-CO2 chambers compared to those growing in the low-nutrient environment of Pellston Plain. Of the five genotypes examined, only one differed in tannin level from its corresponding parent clone. In addition, the mean tannin concentration we measured in June in our low-fertility, ambient-CO2 chamber-grown plants (136 mg/g) was within the range of tannin concentrations reported by Lindroth and Hwang (1996a) from 31 aspen clones sampled during June 1995 on the Pellston Plain (131–273 mg/g).

These results give us some confidence that our data may be useful in predicting future responses by this population to global atmospheric change. While the CNBH accurately predicted changes in leaf condensed tannin concentration, other factors such as genotype and time of year also influenced tannin production. Importantly, the differential response among genotypes to CO2 enrichment suggests that patterns of plant secondary compound production may vary within species as atmospheric CO2 rises, with possible consequences for plant–herbivore and plant–microbe interactions as well as the adaptive response of this species to global climate change.


   FOOTNOTES
 
1 The authors thank Xianzhong Wang, Xiaoqiu Bao, and Kennetha Mays for laboratory and field assistance, Jim Teeri for logistical support, and David Karowe for helpful comments on the manuscript. This work was supported by the University of Michigan Biological Station and by the National Institute for Global Environmental Change. Back

6 Author for correspondence. Back


   LITERATURE CITED
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
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